1. Field of the Invention
The invention relates to an X-ray analysis apparatus, comprising a scanning unit with an X-ray source, a crystal holder which can be rectlinearly displaced in a fixed radiation pick-up direction relative to the X-ray source, an X-ray detection system with an X-ray detector and a motion mechanism for effecting a coupled displacement of the crystal holder and the X-ray detector, the X-ray source, the crystal holder and the X-ray detector remaining on a Rowland circle during said displacement. The invention also relates to a scanning unit suitable for use in an X-ray analysis apparatus such as a simultaneous spectrometer or a sequential spectrometer.
2. Description of the Related Art
A Rowland circle is to be understood to mean herein a focusing circle of a diameter corresponding to the radius of curvature of an analysis crystal situated on said circle, an X-ray source and a detector entrance also being situated on said circle. The choice of the radius R of the Rowland circle is very important in view of the resolution of the apparatus.
An X-ray source is to be understood to mean herein a point on the Rowland circle on which the radiation to be analysed is focused. This point may be situated on a specimen to be analysed, for example in the case where a comparatively small region of the specimen is locally excited by means of an X-ray beam or an electron beam. On the other hand, this point may also coincide with a centre of a slit wherebehind a specimen to be analysed is arranged for excitation. Furthermore, the region to be analysed can be localized by means of a slit on the X-ray detector.
An X-ray analysis apparatus of the kind set forth is known from U.S. Pat. No. 3,123,710. An X-ray analysis apparatus described therein utilizes a scanning unit with a focusing optical system in which, during the measurement process, the crystal is rectlinearly displaced along a fixed line, referred to as the pick-up direction, relative to a fixed point, notably the X-ray source. The detector is at the same time displaced along a path whose origin coincides with the fixed point. In order to sustain the focusing conditions between the X-ray source, the crystal and the detector during the motion, these three components are situated on a focusing circle, i.e. the Rowland circle. In order to ensure that the crystal is rectilinearly displaced in a fixed direction, the Rowland circle should be tilted about the X-ray source during the motion. The choice of the X-ray source as the fixed point is justified by the fact that the specimen to be analysed is thus always analysed with the same pick-up conditions.
This motion is realised by positioning the analysis crystal between the X-ray source and the detector on the Rowland circle, the surface of the analysis crystal being coincident with the Rowland circle. The analysis crystal and the detector are then mechanically interconnected, for example via an anchor-like element as in the cited United States Patent Specification. This element can be driven by means of a linear motion mechanism. In the cited Patent Specification this motion mechanism consists of a worm-wheel-worm drive and a guide.
A drawback thereof consists in the comparatively poor reproducibility and the occurrence of play. Such a drive can be rendered play-free by applying a bias. The reproducibility is closely related to inter alia the magnitude of said bias and the rigidity of the system.